This invention relates to a method of forming a mask for photolithography, and more particularly to a method for forming an attenuated phase-shifting mask (PSM).
An attenuated phase-shifting mask (PSM), a widely used mask in photolithography, is mainly fabricated by coating a phase-shifting layer upon on a transparent plate, thus to interfere the phase of light incident through the transparent plate for providing a better exposure accuracy during the photolithography. As a result of the better exposure accuracy, patterns of attenuated phase-shifting mask would be transformed onto chips more accurate than those of conventional mask do. Typically, a stepper, a machine for projecting rays through masks onto chips during photolithography, identifies and locates a mask by detecting reflected ray form bar codes and position marks at edges of the mask. However, the stepper doesn""t always successfully identify and locate attenuated phase-shifting masks, since a great amount of incident light would pass through the phase-shifting layer so as provide insufficient amount of reflected ray for identifying and locating them. Moreover, patterns of chips transformed form edges of the phase-shifting layer might be exposed in overlap after the use of several attenuated phase-shifting masks. The photoresist layer of those patterns will undesirably lose due to the overlapped exposures, thereby damaging the accuracy of patterns of chips.
For overcoming the above mentioned problems of applying attenuated phase-shifting mask, many semiconductor manufacturers coat an opaque layer around its edges for facilitating steppers to identify and locate the attenuated phase-shifting mask by the reflected ray from the opaque layer at the edges. Referring to FIG. 1, an attenuated phase-shifting mask 34 is coated with a opaque layer 38 around the central image field 36, which includes patterns formed by a phase-shifting layer coated on a transparent plate. Due to the opaque layer 38, a stepper is able to detect the reflected ray from the bar code 40 and position marks 42 to identify and locate the attenuated phase-shifting mask 34.
Through FIG. 2 to FIG. 8, a conventional process for fabricating an attenuated phase-shifting mask with the opaque layer in semiconductor manufacturer""s plant is shown. FIG. 2 shows a cross-sectional view of an attenuated phase-shifting mask 44 provided by mask vendors. The attenuated phase-shifting layer 44 is formed by successively coating a phase-shifting layer 48, opaque layer 50, and first photoresist layer 52 on a transparent plate 46. In order to form patterns on the attenuated phase-shifting mask 44, the first photoresist layer is defined as shown in FIG. 3 by exposure and development. Referring to FIG. 3, parts of the opaque layer 50 are then stripped through a conventional etching process by employing the photoresist layer 52 as an etching mask. Then the photoresist layer 52 is totally stripped thus to shape the cross-sectional view of FIG. 4. Using the opaque layer 50 as an etching mask, parts of the phase-shifting layer 48 are removed until exposing the surface of transparent plate 46, as shown in FIG. 5.
Referring to FIG. 6, a second photoresist layer 54 is blanketed over the transparent plate 46. Through steps of exposure and development, the second photoresist layer 54 is partially removed as the cross-sectional view shown in FIG. 7. Parts of the opaque layer 50 exposed out of the second photoresist layer 54 are stripped through an etching process by employing the second photoresist layer 54 as an etching mask, then removing the second photoresist layer 54 to complete the attenuated phase-shifting mask 44, as shown in FIG. 8. As a result of the opaque layer 50 around upon the phase-shifting layer 48 and transparent layer 46, the stepper may correctly identify and located the attenuated phase-shifting mask 44 by detecting reflected rays from the opaque layer 50.
Although this conventional process enables semiconductor manufacturers to fabricate an attenuated phase-shifting mask with an opaque layer, it exists some shortcomings. This conventional process is much complicated, since it needs to an extra photoresist layer, namely the aforementioned second photoresist layer. For forming the extra photoresist layer, the semiconductor manufacturers need to coat second photoresist layer on a original mask, which already has a first photoresist layer provided by mask vendors. Because of the square shape of mask, it""s hard to coat the second photoresist layer uniformly as its corners by semiconductor manufacturers themselves. Therefore, under this conventional process, the semiconductor manufacturers suffer low yield rates and high costs from it.
In brief, for forming an opaque layer around an attenuated phase-shifting mask as well as avoiding the low yield and cost due to the conventional process, there is huge need to improve the conventional process to overcome the above problems.
An objective of this invention is to provide semiconductor manufacturers a simplified method to from an attenuated phase-shifting mask.
This invention discloses a method for forming an attenuated phase-shifting mask, including following steps. A transparent plate is provided, on which a phase-shifting layer, opaque layer, and undeveloped photoresist layer being stacked on the transparent plate successively. A first part of the photoresist layer is removed until exposing a first region of the opaque layer through exposure and development processes. The first region of the opaque layer is removed for exposing parts of the phase-shifting layer. A solidified thin surface of the photoresist layer is stripped by low-energy plasma treatment. A second part of the photoresist layer is removed for exposing a second region of the opaque layer through exposure and development processes. The exposed phase-shifting layer is then etched by employing the opaque layer as an etching mask. Then the exposed opaque layer and photoresist layer are successively remove, thus forming a complete attenuated phase-shifting mask.